1. Field of the Invention
The present invention relates to a focus offset correction method, more particularly, it relates to a method for determining a focus offset of an optical disk device.
2. Description of the Related Art
An optical information read/write device comprises a focus servo circuit system which irradiates an optical spot on a disk surface to write information data in a high density and read data from the disk. The focus servo circuit comprises a focus error detection circuit which detects the dislocation of the spot from the focal point of the objective lens. The lens is attached to an actuator which drives the lens in a direction perpendicular to the disk surface in response to the output signal from the focus error detection circuit so that the spot on the disk surface is always kept at the focal point of the objective lens.
However, if the lens is inaccurately assembled at a dislocated position, the focal point of the lens is dislocated from the disk surface even when the focus servo control is conducted. Besides, the disk surface is fluctuated during the operation, which increases the dislocation of the focal point. As a result, the information data is not correctly written in the disk and the level of signal for reading the data is lowered, which decreases the reliability of the read/write function of the device.
In order to cope with such problems, for example, Japanese Patent Application Laying Open (KOKAI) No. 61-182638 discloses a focus servo circuit for optical read/write device. According to the proposed circuit of the patent document, an offset is arranged in the focus servo circuit so that the amplitude of the reading signal from the optical disk is maximized.
To detect the amplitude of the reading signal stably and reliably, it is necessary to position the optical spot on the adjusting track and follow the track by a tracking servo control means. In this case, when the focus offset value for maximizing each of the tracking error signal amplitude and the reading signal is far away from zero point, it becomes difficult to accurately carry out the tracking operation or even the servo system becomes out of control since the tracking error signal amplitude in the initial state where the focus offset is zero is far smaller than the optimal value.
In such a case, the tracking has to be rearranged by repeating trial and error by changing the focus offset value, which takes much time.
Also, it is undesirable that the tracking servo system becomes out of control since the actuator of the servo system or optical elements may be damaged by reckless motion of the system.
On the other hand, the above-mentioned patent publication discloses the focus serve circuit wherein the optimal value of the focus offset is determined so as to maximize the tracking error signal amplitude at the time when the tracking serve is off. However, if the optical spot on the disk surface has astigmatism generated therein, the reading signal amplitude does not become maximum when the tracking signal amplitude is being maximum, which makes it difficult to determine the optimal focus offset value.